Chromium plating



Patented Dec. 29, 1931 UNITED STATES A CORPORATION OF MICHIGAN PATENT OFFICE JOHN F. K; MGCULLOUGH AND BENJAMIN W. GILCHRIST, 0F TOETROIT, MICHIGAN,

ASSIGNOBS TO TERNSTED'I" MANUFACTURING COMPANY,

015 DETROIT, MICHIGAN,

ennomuirr marine electrolytic solution and electric current has 5 been proposed years ago. In the early nineties Messrs. Placet and Bonnet disclosed in several foreign and United States patents numerous solutions that were stated to pro-- duce good results. These disclosures are very 10 general. Numerous forms of chromium and other metals are stated in one or another of .these patents to be capable of use, and the.

chromium itself is largely if not wholly confined to chrome sulphates, chrome alums, and chromic acid baths.

Notwithstanding the variety and multiplicity of solutions, baths and acids, which had been stated in the Placet and Bonnet disclosures, practical chromium plating has only in the last few years become active, and thisactivity is now only in the pioneering stage. Great practical difliculty has been found, largely, due to the poor throwing capacity of chromium as compared to nickel and copper, it being found that unless conditions are absolutely right the high spots plate while some of the recesses have little or no coating of chromium. Within the last few years there has been a great deal of investigation and experimental work together with some practical plating, but so far as we are-advised, all these baths had been chromic acid (C10 baths. These baths are all founded onthe so-called Sargent solution, comprising a definite quantity of chromic acid and ;chromi um sulphate. This particular solution has been varied somewhat by the addition of other ingredients, 'and some stress has been placed in some recent patents upon the proper control of the bath, to wit: the'kn owledge of exactly how much of a catalyst is presentdisclosures of Placet and Bonnet concerning Application filed March 26, 1928.; Serial No. 264,952.

the catalyst content of the bath. This literature is here epitomized for the purpose of showing the prevailing practice and the care and nicety which is required in the proper operation of the chromic acid bath, thereby contrasting the wide departure from the pre- .vailing practice which has been accomplished by our bath.

Our bath is founded on the use of chromic chloride and one or more, preferably the three, alkaline metal chlorides,potassium,

sodium and ammonium chloride, with or without other alkaline metal or earth salts such as sodium fluoride, potassium fluoride, ammonium fluoride, or barium, strontium, and calcium fluorides, chlorides, etc. salts raise. the conductivity of the bath and a given voltage will allow the passage of more current, 'thereby plating more rapidly and consequently efiecting economy in plant investment, supervision, etc.

We find it desirable to use a reducing agent for reducing'the chromium compounds from higher states of oxidation to lower states of oxidation. We propose to preferably employ soluble chromium or chromium alloy anodes. We find that this reducing agent is desirable in connection with these chromium anodes for the purpose of reducing the hexavalent form of chromium, such as the chromates, to the chromic stage, trivalent. This reduces the chromates into which compounds, among others, the anodes, dissolve to the same stage as the chromic salt with which the bath started, to wit: the chromic chloride. We find that oxalic acid with hydrochloric acid is a suitable reducing agent for reducing the hexavalent chromium to the trivalent chromium.

Hydrochloric acid in small amounts is always found in a bath of this kind, when chromium chloride is dissolved in water. This will serve to combine with the oxalic acid to reduce the hexavalent form of chromium to the trivalent form. This hexavalent form of chromium exists only when souble' chromium anodes are used. After the bath has These at the anodes.

,It is therefore desirable to add from time to time amounts of hydrochloric acid and oxalic acid to the bath to reduce the hexavalent chromium in the-proportions that are given in the above equation.

We are aware that the use of oxalic acid has heretofore been 'propsed for use in the chromium bath, but it has been proposed for the purpose of acidulating the bath. The purpose of this acidulation is to dissolve the suboxides that are deposited on the cathode. So far as we are aware no acid has been introduced into a chromium bath which has a soluble chromium or chromium alloy anode for the purpose of acting as a reducing agent to reduce the chromates formed by the dissolution of the anode. Certainly no intelligent control and calculation of the bath with this end inview has heretofore been practiced.

We find that an alkaline fluoride such as sodium fluoride is also effective in preventing the precipitation ofthe basic oxides of chromium on the cathode.

The formula that we find very successful is: Chromium chloride (CrCl 6H O) 300 grams per liter to saturation (cold), potassium chloride 250 grams per liter to saturation (cold), sodium chloride 250 grams per liter to saturation (cold), ammonium chloride 150 grams per liter tosaturation sodium fluoride 3 to 5 grams per liter oxalic acid 75 to 100 grams per liter cold), nitric acid 1%.

We find that the nitric acid is desirable in preventing the formation'of the chromium oxide, and to act as a depolarizing agent,

I which facilitates the production of the chromium.

, We prefer to use chromium chloride in a crystalline form, that is, the hydrated chromic chloride which comes in the form of crystals and has the formula CrCl 6H O. We use the violet modification of this chromic chloride. It is our understanding that when chromic chloride is dissolved in water three different modifications .result, namely: Beta chromic chloride (Cr(H O)6) C1 violet; alpha chromic chloride I '(Cr(H O4Cl )Cl+2H O, green in color, and gamma chromic chloride (Cr(H O)5Cl )Cl+H O, also greenjn color. With the violet salt, the solution must be nearly neutral and' the temperature must rfnot be too high,from 80 to 140'degrees Fahrenheit. The acidity is regulated by the addition of caustic soda.-

With our bath we use a voltage of from 8 to 12, and a current density of from 250 to This reaction is as follows:

cold) cold chlorides, in particular, is useful in eliminating the emission of noxious orpoisonous gases. This is especially true in avoiding the use of chromium sulphates, including a chromium alum bath. The reaction in such a bath results in the formation of chromic acid. We find it desirable to eschew the joint use of -the alkaline chlorides and alkaline sulphates, or any combination of-the members of the two groups. These are bound to result in the production of chromic acid. Where chromic acid is formed in a bath of this kind, it reacts with the salts in the bath and free chlorine gas is given off at the anode.

' This is extremely poisonous. We also find that it is desirable to use a chromium or a soluble chromium alloy anode, even where only the chloride salts are used. Where insoluble anodes are used chlorine gas is released, which does not combine at the anode as is the case where a soluble chromium anode is used.

The voltage of 8 to 12, and the amperage of 250 to 1000 is stated at room temperature. It is of course well understood that when the temperature of the bath rises there will be a larger amperage d a smaller voltage. Therefore, when we state our limits of voltage and amperage in the claims, it is understood that this is a relative statement at room temperature, and that this is intended to cover other limits which would be automatically secured by varying the temperature of the a bath. It will be apparent that the current density is dependent upon several factors,- the cathode area, the temperature of the bath, the distance between electrodes, voltage,

in part and a substitute for application Serial No. 198,689, filed June 13, 1927; What we claim is:

1. A process for chromium plating which comprises passing an electric current through a bath with a chromium anode and formed by dissolving in water chromic chloride and one or more of the alkaline metal and earth metal chlorides, and oxalic acid together with nitric acid. v 2. An electro-plating bath in which chrmium chloride has been dissolved in water,

together with potassium chloride, sodium chloride and ammonium chloride, and substantially the proportions of 300 grams per liter to saturation of the chromium chloride,

250 grams per liter to saturation of the potas-. sium chloride, 250 grams per liter to saturation of the sodium chloride, and 150 grams per liter to saturation of the ammonium chloride,'together with 3 to 5 grams per liter of the sodium fluoride.

3. An electro-plating bath in which chromium chloride has been dissolved in water,

together with potassium chloride, sodium chloride and ammonium chloride, and substantially the proportions of 300 grams per liter to saturation of the chromium chloride, 250 grams per liter to saturation of the potassium chloride, 250 grams per liter to saturation of the sodium chloride, and 150 grams per liter to saturation of the ammonium chloride, together with substantially 3 to 5 grams sodium fluoride per liter, to grams oxalic acid, and 1%.nitric acid.

In testimony whereof we have afiixed our signatures. JOHN- F. K. MCCULLOUGH.

BENJAMIN W. GILCHRIST. 

